1. Field of the Invention
This invention relates to a cryogenic refrigerator having a dual circuit comprising a precooling refrigerating circuit for expanding refrigerant gas, such as helium gas, and a J-T (Joule-Thomson) circuit, in which a cryogenic working apparatus is maintained at a very low temperature level by generating cold at a cryogenic level maintaining part in a cryostat (cryogenic tank), and in particular to a measure for reducing vibration while cryogenic working apparatus is used.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,223,540, a helium refrigerator is well known as a very low temperature refrigerator. This helium refrigerator is provided with a precooling refrigerating circuit, whereby a cryogenic maintaining part in a cryostat is radiantly shielded from the outside by expanding high pressure helium gas by an expander, and a J-T circuit whereby compressed helium gas discharged from another compressor is precooled in said precooling refrigerating circuit and such precooled helium gas is then Joule-Thomson expanded at a J-T valve to generate cold in the cryogenic level maintaining part of the cryostat by expanding action at that time.
In such a helium refrigerator as mentioned above, the G - M cycle (Gifford-MacMahon cycle), the modified Solvay cycle of the like is generally employed as a refrigerating cycle produced by a precooling refrigerating circuit. In this case, it is inevitable that vibration is generated due to a change of pressure in gas flowing at an expander, collision of a displacer with a cylinder, expansion and shrinkage of a cylinder due to change of pressure (high pressure/ low pressure), etc. Thus, it was difficult to use such cycles in a system using a photo-detecting sensor to be used in spectrochemical study where micro vibration in the order of .mu. m order must be avoided. Therefore, when using such a photo-detecting sensor, the operation of a refrigerator is stopped and measuring is finished by utilizing thermal capacity at a sensor part and at a heat station before the temperature at the sensor part rises beyond the temperature required for cooling the sensor part. However the, stoppage of a refrigerator during the operation of a sensor causes the following problems.
When a refrigerator is working, helium gas feeding pressure and return pressure are maintained at about 20 atm, and 1 atm respectively, and helium gas which passed through a J-T valve is partly liquefied and is maintained at a very low temperature level of about 4K. However, as soon as the refrigerator is stopped, pressure in the J-T circuit is balanced at about 8 atm and as shown in FIG. 11, liquid helium at the sensor part reaches a supercritical pressure in a moment and its temperature rises to 5.5-8K.
Generally, when a very low temperature level is reached, thermal capacity at each part becomes small due to small specific heat and even the slightest thermal load causes an abrupt rise in temperature.
Therefore, in the sensor which utilizes the phenomenon of super conductivity or which is reduced in low heat noise, the sensor temperature rises abruptly and as a result problems, such as the breaking down of the superconductivity the difficulty in measuring due to the increase of heat noise, etc., are raised.